A toxic by-product of the Bayer process, red mud has proven problematic in industry for several decades. The substance is generated as a waste product during the production of alumina. For every tonne of alumina produced, 1 to 2 tonnes of red mud are created as a waste stream. Current estimates put the global red mud stocks at 2.5 to 3 billion tonnes. World output is estimated at 50 million tonnes per annum, with Australia the biggest contributor (its output is up to 25 million tonnes per annum). It is estimated that demand for alumina will increase over the following decades, from a combination of developing economies and emerging technologies. The problem of what to do with red mud is therefore a major issue.
The toxic effects attributed to red mud are due to its highly caustic (i.e. basic) nature. During the initial stage of traditional alumina production, bauxite is reacted with concentrated sodium hydroxide (caustic soda) at temperature. The tailings discarded contain a high proportion of caustic soda, in both a raw and chemically combined form. The red mud ‘residue’ is therefore highly corrosive to flora, fauna and the environment, and must be partially treated to enable safe and responsible storage. Current methods employ partial washing of the toxic residue to decrease the caustic nature, prior to storage in evaporating ponds or dry stacks. These are problematic in themselves; evaporating ponds ultimately invade and react with the surrounding ecosystem, while dry stacks have to be constantly maintained to limit caustic run-off and surface dusting. Spillages of red mud have resulted in human fatalities in addition to damage to rivers, ecosystems, buildings and homes. A viable solution to the treatment and/or reclamation of red mud is required.
Previous methods employed for the treatment and/or reclamation of red mud have attempted to deal with the toxicity either by partial neutralisation or further processing. Queensland Alumina Limited have used seawater for partial treatment, reducing the caustic nature of the residue prior to storage. However, such methods do not fully eliminate the apparent dangers with the substance, nor utilise the entirety of the waste. Other methods have taken the route of metal recovery from the waste. For example, reclamation of iron oxides has proven successful, as has recovery of titanium employing sulphuric acid leaching followed by magnetic separation. Alcoa have developed a process which involves breaking down red mud into components suitable for various applications; their Red Sand, Red Lime and Alkaloam products have uses in areas such as agriculture, acid mine drainage and catalysis. Yet other approaches use neutralised red mud for building materials, ceramics, waste-water treatment and other novel applications. The major drawback of most of these routes is the inability to utilise the majority of the red mud waste. Various value-added metals are entrapped within the residue and, to date, no process has been developed which utilises most of the entrapped valuable materials. The typical composition of red mud contains up to 25% alumina, 60% iron and 15% silica. Other metals such as titanium, magnesium and gallium are also present in trace amounts.
There is a need for processes that process red mud to provide valuable materials and/or overcome the difficulties associated with the storage of red mud.